This invention relates to a method for measuring a gloss profile of the surface of such materials as paper, synthetic resin films, metals, and the like.
Various materials have a gloss, and, in most cases, it is desirable that the gloss is uniform. The gloss profile, depending on its degree, may affect the product values.
In paper, for example, the apparent evenness of its surface is called a "sheet appearance," which is a very important factor for the evaluation of paper quality. The sheet appearance is substantially evaluated in terms of the uniformity of whiteness and gloss of the paper surface and the paper thickness. If there are any unevenness in these factors, the paper is evaluated to have a low sheet appearance. The cause for surface unevenness of paper includes fluctuations in the basis weight and caliper during the paper manufacturing process, which leads to a low sheet appearance. As for coated paper, which is coated on the surface with pigments, the purpose of coating is to improve the surface smoothness, brightness and opacity. Therefore, an uneven gloss profile means an imperfect coating purpose, resulting in a defective product. In particular, "orange peel," a fine unevenness that occurs in the coated surface, is greatly related with the gloss profile.
Under such circumstances, the presence and degree of gloss profile in coated paper has a great significance in the process control of the coated paper production. However, in an actual process control, the gloss profile is not directly evaluated, but the emphasis of the evaluation is placed more on the sheet appearance. However, the evaluation depends greatly on the inspector's subjective, less consistent evaluation criteria. Therefore, a more objective and exact evaluation method has been in demand.
Heretofore, there has been used a surface roughness tester as a method for measuring the surface roughness by a physical contact. This method is effective for measuring a roughness of the micron level, however, since fine surface irregularities have a smaller pitch than that detectable as gloss profile by the human eye. Therefore, it is not directly connected with gloss profile.
Further, Japanese Patent Publication 1-24256 discloses a three-dimensional optical device comprising a light source and a light receiver, capable of varying the incident angle and the reflection angle of the sample. This device is used to measure the tone of color from the brilliance and brightness of a metallic coating.
Since gloss profile is a fluctuation condition of glossiness, it can be determined as a distribution of glossiness. Therefore, it is possible to measure the gloss profile using the above-described three-dimensional optical device. However, since this optical device uses a tungsten lamp as a light source, filtered to obtain a white light, it is suitable to measure the tone of color, but is not always suited for measuring the gloss profile.
With a view toward obviating above prior art defects, it is a primary object of the present invention to provide a method for measuring a gloss profile of the sample surface, more specifically, a simple method for measuring a sheet appearance of coated paper rapidly and accurately.